$ sudo tcpdump -n -i enp0s25 -c 5 host 239.255.42.42; sudo iptables -S; ffmpeg -i 'udp://239.255.42.42:5004?timeout=10000000'; sudo tcpdump -n -i enp0s25 -c 5 host 239.255.42.42
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on enp0s25, link-type EN10MB (Ethernet), capture size 262144 bytes
13:13:41.807541 IP 192.168.1.238.5004 > 239.255.42.42.5004: UDP, length 1316
13:13:41.807804 IP 192.168.1.238.5004 > 239.255.42.42.5004: UDP, length 1316
13:13:41.808322 IP 192.168.1.238.5004 > 239.255.42.42.5004: UDP, length 1316
13:13:41.808662 IP 192.168.1.238.5004 > 239.255.42.42.5004: UDP, length 1316
13:13:41.808920 IP 192.168.1.238.5004 > 239.255.42.42.5004: UDP, length 1316
5 packets captured
7 packets received by filter
0 packets dropped by kernel
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
ffmpeg version 3.3.4 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 7.2.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-avisynth --enable-avresample --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libass --enable-libbluray --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxvid --enable-shared --enable-version3
  libavutil      55. 58.100 / 55. 58.100
  libavcodec     57. 89.100 / 57. 89.100
  libavformat    57. 71.100 / 57. 71.100
  libavdevice    57.  6.100 / 57.  6.100
  libavfilter     6. 82.100 /  6. 82.100
  libavresample   3.  5.  0 /  3.  5.  0
  libswscale      4.  6.100 /  4.  6.100
  libswresample   2.  7.100 /  2.  7.100
  libpostproc    54.  5.100 / 54.  5.100
udp://239.255.42.42:5004?timeout=10000000: Input/output error
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on enp0s25, link-type EN10MB (Ethernet), capture size 262144 bytes
13:13:52.601117 IP 192.168.1.238.5004 > 239.255.42.42.5004: UDP, length 0
13:13:52.602149 IP 192.168.1.238.5004 > 239.255.42.42.5004: UDP, length 0
13:13:52.603126 IP 192.168.1.238.5004 > 239.255.42.42.5004: UDP, length 0
13:13:52.604257 IP 192.168.1.238.5004 > 239.255.42.42.5004: UDP, length 0
13:13:52.604515 IP 192.168.1.238.5004 > 239.255.42.42.5004: UDP, length 1316
5 packets captured
6 packets received by filter
0 packets dropped by kernel